This invention pertains to determining oxygen-induced stacking fault density of wafers, and more particularly, to predicting extrinsic backside oxygen-induced stacking fault density by measuring surface roughness of a roughened wafer surface.
Wafers used in the microelectronic industry, such as silicon (Si) and gallium arsenide (GaAs) wafers, are manufactured under stringent quality control with regards to contamination. During the manufacturing process, though, it is inevitable that contamination by elements will occur resulting in the wafer having undesirable electrical properties, possibly rendering a portion or the entire wafer useless. Transition metal contamination is of significant concern. The metal impurities can exist in a dissolved or precipitated state within the wafer, both of which produce deleterious effects. For example, dissolved transition metals can enhance carrier recombination which increases the background noise of a charge-coupled device (CCD) and increases the base current in bipolar devices. Additionally, metal impurities migrate to or precipitate at crystal interfaces which can degrade the dielectric strength of thin oxide layers, leading to gate oxide integrity (GOI) failures.
Removal of impurities from the active regions of the wafer by attracting the impurities to another region is generally referred to as gettering. One form of gettering is accomplished through a phenomena wherein crystal structural defects internal to the wafer act as precipitation sites for metal impurities. Some of these structural defects are oxygen precipitates and growth-related defects such as oxygen-induced stacking faults (OSF) and dislocation loops. During subsequent high temperature processes, the atoms of the metal impurities will migrate to the OSF locations and remain held there thus purifying the outer surfaces of the wafer. This type of gettering is generally referred to as intrinsic or internal gettering since it occurs with intrinsic OSF.
Another technique used to produce gettering properties that is particularly useful in preventing contamination from external sources as well as internal sources is to intentionally create extrinsic OSF. Extrinsic OSF relies on the phenomena whereby OSF will form in an oxidizing environment at crystal structure defects on a surface of the wafer. The active region of the wafer is referred to as the frontside; the region wherein electronic devices are produced. Extrinsic OSF is therefore typically produced on the backside of the wafer to draw the impurities away from the frontside.
One method of producing crystal defects which, in turn will produce OSF, is by producing backside damage (BSD) on the wafer. Wet sand blast (WSB) processes are known in the art to produce crystal defects in the form of surface roughening by blasting, at a predetermined pressure, a slurry of fine silicon dioxide powder and water directed against the backside surface of the wafer.
In subsequent oxidizing high temperature processes, OSF are formed at the defect locations. As a result, the impurities within the wafer migrate to the backside of the wafer and any external impurities, such as those found in high temperature furnaces, are attracted to the backside rather than depositing on the frontside of the wafer.
There is a balance between too little OSF to produce an effective gettering effect and too much OSF that renders the wafer with unacceptable electrical and structural properties. Therefore, in wafer production it is important to periodically monitor the effectiveness of the surface damaging process. A unit of measure used in the art is OSF density; that is, how many OSF""s are present in a given surface area of the wafer. One crystal defect will generally correspond to one OSF.
One method of monitoring the effectiveness of the damage caused by the surface roughening process includes processing one or more monitor wafers along with the wafers of a production run of a particular shift. After the surface roughening process, the monitor wafer is removed from the production run and processed in a high temperature oxygen rich furnace to oxidize the damaged surface thereby creating OSF. Since the size of individual OSF is on the order of a few xc3x85rms and difficult to measure using visual methods, additional surface modification is required. The damaged surface is exposed to a chemical etchant that selectively etches and enlarges the OSF sites forming enlarged OSF defects that can be detected visually under magnification.
A microscope is used for direct or photographic inspection of various locations of the damaged surface of the wafer. In one method, three standard locations on the damaged surface are inspected wherein the surface roughness is quantified by counting the visible defects in a predetermined area to determine defect density. A known linear relationship has been established that correlates OSF density wide defect density (U.S. Pat. No. 6,275,293, Brown, incorporated herein by reference). An average of the defect density at the standardized locations is used to determine the overall OSF density, which is extrapolated to the batch of wafers from which the monitor wafer was pulled.
The above process, though currently used in the industry, has a number of disadvantages. The process requires processing the monitor wafer in an oxidizing furnace, which is time consuming and contributes to testing variability. The wafer is etched in hazardous and toxic chemicals raising safety and environmental issues, and may lead to variability due to chemical concentration and temperature changes. The defects are manually counted which is a slow and tedious process that is prone to human error and lack of uniformity from one operator to the next. Only certain locations on the surface are inspected which provides limited information regarding the uniformity of the surface defects across the entire wafer. Also, the process is a destructive test wherein the monitor wafer is scrapped after evaluation.
It is common for this evaluation process to take two to three days to complete, by which time the production wafers have undergone extensive and expensive finishing steps and are awaiting release for shipment. If a quality issue arises, it is possible that all of the wafers produced in the particular shift will have to be scrapped at a significant cost to the producer. Also, if the production parameters have not been changed, wafers produced in subsequent shifts might be at risk of being scrapped, until the parameters can be adjusted.
Therefore, methods and apparatus are needed to determine OSF density of wafers that address these issues.
The present invention provides methods and apparatus for predicting the density of oxygen-induced stacking faults (OSF) on a surface of a wafer by measuring the surface roughness before and after a surface damaging process. In one embodiment of the method, surface damage is produced by a wet sand blast (WSB) process. The surface roughness resulting from the surface damaging process is quantified and compared with the pre-damaged surface roughness data. The difference between the pre- and post-damaged surface roughness data is determined and correlated with oxygen-induced stacking fault density to surface roughness correlation data to obtain the predicted oxygen-induced stacking fault density.
In another embodiment in accordance with the invention, automated computer-assisted wafer OSF density evaluation apparatus is provided that receives the wafer, measures the surface roughness over at least a portion of the surface, correlates the data with a predetermined database of OSF density data, and presents the predicted OSF density for that particular wafer.
The methods in accordance with the present invention replace, among other things, the wafer oxidizing and etching process. These methods address the variability in data resulting from manual inspection and counting, as well as furnace and chemical etchant fluctuations. The wafer quality evaluation can be obtained within hours rather than days, providing for a quick response to quality deviations prior to further finishing processes of the production wafers. Surface roughness variability is more completely and accurately determined since substantially the entire wafer surface can be evaluated. Further, since the OSF""s have not been enlarged by an etching process, after the evaluation, the monitor wafer can be re-polished and reused, resulting in significant savings to the producer.
Further, the methods allow for an automated quality-monitoring process using surface-scanning machinery and computer-stored databases of surface roughness to OSF correlation data. Automation of the process is possible as a number of difficult to automate processes, including high-temperature oxidation followed by chemical etching, are eliminated. Further, the automated process removes the variability introduced by labor-intensive manual visual inspection under magnification and operator interpretation of the results.
These and other variations as well as the invention itself will become more readily apparent upon reference to the following detailed description that follows.